In a typical radio communications system, user communications terminals referred to as user equipment units (UEs) communicate via a radio access network (RAN) with other networks like the Internet. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “NodeB” or enhanced Node B. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site.
Third Generation (3G) cellular radio systems like Universal Mobile Telecommunications System (UMTS) operating in Wideband Code Division Multiple Access (WCDMA) use different types of radio channels including unscheduled radio channels and scheduled radio channels. Mixed voice/data, circuit/packet switched 3G systems evolved from voice-centric, circuit-switched second generation (2G) systems. Unscheduled channels, sometimes called dedicated channels, are usually allocated to only one user for the duration of a connection carrying information only associated with that one user. Scheduled channels are packet-switched channels over which packets for multiple user connections are carried. Fourth generation (4G) systems, like the Long Term Evolution (LTE) of UMTS and Worldwide Interoperability for Microwave Access (WiMAX), design the air interface based on packet data. Dedicated traffic channels are eliminated in favor of scheduled radio channels in order to simplify the system. Medium access control is thus migrating towards a request resource-grant resource paradigm. In response to actual requests to transmit data from and/or to a user equipment (UE) in the uplink and/or the downlink, the scheduler in the base station dynamically allocates radio resources to satisfy the quality of service requirements associated with the type of data traffic to be transmitted, and at the same time, tries to optimize the system capacity.
FIG. 1 illustrates an example of an LTE type mobile communications system 10. An E-UTRAN 12 includes E-UTRAN NodeBs (eNBs) 18 that provide E-UTRA user plane and control plane protocol terminations towards the user equipment (UE) terminals 20 over a radio interface. An eNB is sometimes more generally referred to as a base station, and a UE is sometimes referred to as a mobile radio terminal or a mobile station. As shown in FIG. 1, the base stations are interconnected with each other by an X2 interface. The base stations are also connected by an S1 interface to an Evolved Packet Core (EPC) 14 which includes a Mobility Management Entity (MME) and to a System Architecture Evolution (SAE) Gateway. The MME/SAE Gateway is shown as a single node 22 in this example and is analogous in many ways to an SGSN/GGSN gateway in UMTS and in GSM/EDGE. The S1 interface supports a many-to-many relation between MMEs/SAE Gateways and eNBs. The E-UTRAN 12 and EPC 14 together form a Public Land Mobile Network (PLMN). The MIMEs/SAE Gateways 22 are connected to directly or indirectly to the Internet 16 and to other networks.
The IEEE 802.16 Working Group on Broadband Wireless Access Standards develops formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks (MAN). Although the 802.16 family of standards is officially called WirelessMAN, it is often referred to as WiMAX. Like LTE, WiMAX/IEEE 802.16e uses scalable orthogonal frequency division multiple access (OFDMA) to support large channel bandwidths, e.g., between 1.25 MHz and 20 MHz with up to 2048 sub-carriers for WiMAX. Another important physical layer feature is support for multiple-in-multiple-out (MIMO) antennas in order to provide good NLOS (non-line-of-sight) characteristics (or higher bandwidth). Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver use multiple antennas resulting in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
One working assumption in LTE relating to MIMO is the support of a spatial multiplexing mode with channel-dependent (closed-loop) precoding. A precoder maps data symbols to be transmitted onto all of the multiple transmission antennas. Different precoders map the symbols in different combinations onto to each antenna. The spatial multiplexing mode achieves higher data rates in favorable channel conditions.
LTE may also support a spatial multiplexing mode with channel-independent (open-loop) precoding in the form of precoder cycling. An example illustration of a MIMO communication model that uses precoder cycling is shown in FIG. 2. Here, the transmitter cycles through four precoders W1-W4 to precode different sets of four baseband symbol vectors to be transmitted, e.g., s1-s4, s5-s8, etc. The precoders, W1-W4, map the symbol vectors, s1-s4, s5-s8, etc. to precoded baseband symbol vectors, x1-x4, x5-x8, etc. through a matrix-vector multiplication operation, e.g., x1=W1s1. The elements of a precoded baseband symbol have a one-to-one correspondence to the transmit antenna ports. Each precoded baseband symbol vector is thereafter transmitted over one of the effective MIMO channels, H1-H4, H5-H8, etc. An “effective MIMO channel” models the physical radio communications channel along with the physical antennas, radio hardware, and baseband signal processing used to communicate over that channel. Thus, several different communication techniques, e.g., OFDM and CDMA are illustrated as examples in FIGS. 8 and 9 as explained below, may be represented using this same effective channel model.
Cycling is achieved by precoding one symbol s1 with precoder matrix W1, symbol s2 with precoder matrix W2, symbol s3 with precoder matrix W3, and symbol s4 with precoder matrix W4, and then using W1-W4 to precode the next four symbols and so forth. The receiver receives parallel signals y1-y4, y5-y8, etc., and filters them in respective filters f1-f4, f5-f8, etc. modeled based on the four precoders W1-W4 to produce estimates ŝ1-ŝ4, ŝ5-ŝ8, etc. of the symbols s1-s4, s5-s8, etc. originally transmitted. Alternatively, the receiver detects the bit-streams represented by the symbols s1-s4, s5-s8, etc. directly from the received parallel signals y1-y4, y5-y8, etc. using maximum-likelihood decoding (or some other decoder metric).
An example illustration of a transmission structure 30 for implementing a precoded spatial multiplexing mode is provided in FIG. 3. A data stream corresponds to a MIMO “layer” 12, and each layer 12 provides one symbol s at a time to a precoder 34. The parallel symbol output from all of the MIMO layers corresponds to a symbol vector s, which is multiplied in the precoder 34 by an NT×r precoder matrix WNdi T×r , which serves to distribute the transmit energy substantially in a subspace of the NT dimensional vector space, where NT is the number of transmit antennas. If the precoder matrix 34 is confined to have orthonormal columns, then the design of a codebook of precoder matrices corresponds to a Grassmannian subspace packing problem. Each of the r symbols in symbol vector s corresponds to a MIMO layer, and r is referred to as the “transmission rank.” Spatial multiplexing is achieved by transmitting the precoder outputs via inverse fast Fourier transformers (IFFTs) 36 used in orthogonal frequency division multiplexed (OFDM) transmissions, where multiple symbols are transmitted simultaneously over the same transmission resource element (RE). The IFFT 36 outputs are transmitted via NT antenna ports 38. In the case of OFDM, the same RE corresponds to the same frequency subcarrier or “bin.” The number of parallel symbols r may be adapted to the current communications channel properties.
Based on the model in FIG. 2, a received NR×1 vector yk for a certain resource element on frequency subcarrier k (or alternatively data RE number k), assuming no inter-cell interference, is represented for each subcarrier k by:yk=HkWNT×rsk+ek  (1)where Hk represents the effective MIMO communications channel, WNT×r is an NT×r precoder matrix, sk is an r×1 symbol vector, and ek is a noise vector obtained, e.g., as realizations of a random process.
The precoder matrix 34 may be chosen to match the characteristics of the overall NR×NT MIMO channel H (made up of multiple individual MIMO channels H1-H8 etc.), resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially aims to focus the transmitted energy into a subspace that conveys much of the transmitted energy to the UE, rather than “waste” transmitting the signal in areas where the UE is not located. In addition, the precoder matrix may also be selected to orthogonalize the channel, meaning that after linear equalization at the UE receiver, inter-layer interference (interference between different MIMO layers) is reduced.
In closed-loop precoding, the UE transmits a feedback signal, based on channel measurements in the downlink, with recommendations to the base station of a precoder to use that is well-suited to the current channel measurements. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g, several precoders, one per subband. Subspace refers to spatial dimensions and bandwidth to frequency which may be divided into subbands. The appropriate precoder typically varies with frequency (subband). Hence, having a precoder per subband rather than one for all subbands (wideband) enables better precoder adaptation.
A problem with closed-loop precoding is that it takes time to convey the UE's precoder report to the base station, and during that time, the channel may have changed (e.g., faded) significantly making the report outdated by the time the base station has a chance to apply it. Thus, closed-loop precoding is more suitable for low mobility scenarios where the channel variations are slow. An exception to this is if the channel exhibits long-term properties that can be exploited even though the mobility is high. Spatial correlation on the base station side is one example of such a property that is relatively stable despite high UE velocities.
In LTE, the encoded bits originating from the same block of information bits is referred to as a “codeword” (CW). A codeword is also the terminology used to describe the output from a single hybrid ARQ (HARQ) process serving a particular transport block and comprises turbo encoding, rate matching, interleaving, etc. The codeword is then modulated and distributed over the multiple transmit antennas. In multi-codeword transmission, data may be transmitted from several codewords at the same time. The first (modulated) codeword may for instance be mapped to the first two antennas, and the second codeword may be mapped to the two remaining antennas in a four transmit antenna system. But in the precoding context, the codewords are mapped to layers, and the precoder maps the layers onto the antennas.
For high rate, multi-antenna transmission, an important characteristic of the channel conditions is the channel rank (which is different from transmission rank). Roughly speaking, the channel rank can vary from one up to the minimum number of transmit and receive antennas. Taking a “4×2” system as an example, i.e., a system with four transmit antennas and two receive antennas, the maximum channel rank is two. The channel rank varies in time as fast fading alters the channel conditions. Moreover, channel rank determines how many MIMO layers/data streams, and ultimately also how many codewords, can be successfully transmitted simultaneously. Hence, if the channel rank is “one” when two codewords mapping to two separate MIMO layers are being transmitted, there is a strong likelihood that the two signals corresponding to the codewords will interfere such that both of the codewords are erroneously detected at the receiver.
In conjunction with precoding, adapting the transmission to the channel rank involves using as many data stream layers as the MIMO channels can support. In the simplest case, each MIMO layer corresponds to a particular antenna. But the number of codewords may differ from the number of data stream/layers, which is the case in LTE. The issue then arises of how to map the codewords to the data stream layers. Assuming four transmit antennas as an example, the maximum number of codewords is two while up to four layers can be transmitted.
A fixed rank dependent codeword-to-layer mapping with precoding for this non-limiting example is shown in FIG. 4. Codewords may be provided from an error correction encoder such as a turbocoder. For channel rank 1, corresponding to one layer or one data stream represented as a codeword (CW 1), the precoder 40 maps the single codeword CW 1 to the four transmit antennas. For channel rank 2, corresponding to two layers or two data streams represented as two codewords (CW 1 and CW 2), the precoder maps the two codewords to the four transmit antennas. For channel rank 3, there are two codewords (CW 1 and CW 2), and the second codeword CW 2 is split via a serial-to-parallel converter (SIP) 42 into two data streams/layers. So the precoder 40 maps the three data streams/layers generated from the two codewords to the four transmit antennas. The second codeword need not be the same length as the first codeword and may for example be twice as long as CW 1. For channel rank 4, there are two codewords (CW 1 and CW 2), and both are split via a corresponding serial to parallel converter (SIP) 42 into two data streams/layers. So the precoder 40 maps the four data streams/layers generated from the two codewords to the four transmit antennas.
Since closed-loop precoding often is not suitable for high mobility scenarios where the channel lacks significant long-term properties and is rapidly changing, an alternative is to select a transmission scheme that is independent of the channel realizations. Such channel independent transmission is also known as open-loop transmission and is more suitable for higher mobility situations. An example open-loop transmission scheme for two transmit antennas is an Alamouti code, which has a counterpart in the frequency domain called a space frequency block coding (SFBC). SFBC takes two symbols sk and sk+l at a time as input and distributes these symbols over frequency and space as described by the codeword matrix:
                    [                                                            s                k                                                                    s                                  k                  +                  1                                                                                                        s                                  k                  +                  1                                c                                                                    -                                  s                  k                  c                                                                    ]                            (        2        )            where the rows correspond to the different antenna ports, the columns correspond to the subcarrier dimension, and ( )c denotes complex conjugate. Typically two consecutive subcarriers are chosen and, without loss of generality, this will be assumed below. So two potentially complex-valued symbols are transmitted using two subcarriers/REs. The symbol rate per RE is thus 1 corresponding to a transmission rank of one and hence suitable for rank one type channels. The above code belongs to the class of orthogonal space-time block codes (OSTBC). The time dimension can be interchanged with another dimension, for example frequency, as is often the case in OFDM. Nevertheless, such codes are referred to here as OSTBC even though they may use a dimension other than time. OSTBC codes exist for more than two transmit antennas as well, but they are typically limited in symbol rate targeting symbol rates (per RE) of one. For 4 transmit antennas, LTE has adopted a combination of SFBC and antenna switching, corresponding to a block code with the following codeword matrix:
                    [                                                            s                k                                                                    s                                  k                  +                  1                                                                    0                                      0                                                          0                                      0                                                      s                                  k                  +                  2                                                                                    s                                  k                  +                  3                                                                                                        s                                  k                  +                  1                                c                                                                    -                                  s                  k                  c                                                                    0                                      0                                                          0                                      0                                                      s                                  k                  +                  3                                c                                                                    -                                  s                                      k                    +                    2                                    c                                                                    ]                            (        3        )            Even though the above code is not an OSTBC code in a strict sense, this code has a symbol rate of one and is thus suitable for rank one type channels.
Open-loop precoding using a higher transmission rank than one is also possible. But because there is no accurate information about the channel, the precoder cannot be matched to the channel. Hence, it would be beneficial to try and achieve precoding diversity to ensure generally acceptable precoder performance over a wide range of different channel conditions.
Another way to introduce open-loop precoding transmission is to reuse the purely spatial precoding structure, where a precoder multiplies a single symbol vector, which is equivalent to each symbol being multiplied by the corresponding column vector in the precoder matrix. In order to achieve precoding diversity, it is important to avoid using only a single precoder since such a transmission only suits a limited set of channel realizations. Accordingly, a single codeword can be transmitted in such a way so that multiple precoders are used, where the precoders are varied in some deterministic manner known to both the transmitter and the receiver. For example, the precoder may be fixed for one or several subcarriers and then changed for a next subcarrier(s). This distributes the energy spatially in a more isotropic manner, (i.e., more towards an even energy distribution in all directions), which provides diversity thereby reducing the tendency to bias the performance for a particular set of channel realizations. Preferably, there should be a substantial precoding variation over the smallest allocation unit, e.g., a resource block (RB), because a codeword may potentially only span a small set of REs.
This can be accomplished using “precoder cycling,” as illustrated in FIG. 2, where the precoder varies from one consecutive set of subcarriers to the next. The precoders that are cycled through are predetermined or configured by the transmitter. For UEs that also have an implementation of a closed-loop precoder scheme, it is advantageous to reuse the precoders in the closed-loop precoder codebook in the open-loop scheme because then significant parts of the UE implementation can be reused for the open-loop precoding scheme.
One problem with open loop, configurable precoder cycling is that the receiver does not know and cannot accurately predict the interference it needs to reject at a particular instant. As the number of cycling precoders increases in an open loop system, it becomes increasingly difficult for the receiver to know or predict which one of the precoders is currently being used in the interfering transmissions. As a result, the receiver is uncertain of the interference changes over a radio block, and thus, does not satisfactorily suppress that interference. Another problem relates to undesirable complexity in the receiver (e.g., a UE). Using many different precoders in the precoder-cycling has the disadvantage of a high implementation complexity (and thus increased power consumption) at both the transmitter and receiver because the precoder operation and receive filtering operation must be implemented and matched to each used precoder. Also, having a configurable number of precoder matrices to cycle over means that the transmitter and receiver must be implemented to cope with the most computationally demanding scenario.